POLYESTERS OF REDUCED VI, AND OF REDUCED TENOR OF ACETALDEHYDE, PREFORMED, AND CONTAINERS OBTAINED WITH A POLYMER OF THIS TYPE The present invention relates to polyester resins, more particularly to polyethylene terephthalate resins used in the manufacture of hollow containers, such as bottles for the packaging of liquid materials, especially soft drinks, natural or mineral waters. For some tens of years, polyester, more particularly polyethylene terephthalate, better known under the abbreviation PET, finds an increasingly important application in the field of the manufacture of hollow containers, and more particularly bottles. Among the limitations imposed on PET for use in this field, the transparency of the containers obtained is one of the most important. They have been proposed many years ago and especially through the European Patent 41035, some. copolymers comprising mainly recurring units of ethylene glycol terephthalate, but also other recurring units derived from the presence of monomers other than terephthalic acid and ethylene glycol. Said monomers are referred to as "crystallization retarders" and are present in the polyester with varying concentrations, for example between 3.5% and 7.5% of all diacid monomers. The other limitation that must be respected by polyester is the content of acetaldehyde in the walls of the container. Indeed, acetaldehyde is a product that results from the degradation of polyester and is formed at an elevated temperature during the course of polyester manufacture and the process of bottle manufacture, especially during the injection stage of the bottles. pre-forms. Numerous processes have been described in numerous patents for obtaining a polyester with a reduced content of acetaldehyde, or methods for limiting its formation during the stages of shaping. One of the techniques used industrially and especially described in the European Patent 41035, is that which consists in carrying out a polycondensation of polyester in a molten medium to a limited degree of polymerization, in order to avoid the formation of large quantities of products. of degradation that are generally acetaldehyde generators when the polyester is heated to temperatures higher than for example 200 degrees centigrade. The polyester. thus obtained is subjected to a new polymerization in solid phase, generally referred to as "postcondensation in solid phase", carried out at a lower temperature, in the order of 200 to 220 degrees centigrade, usually under a nitrogen atmosphere. During the course of said operation, the polyester is polycondensed to a degree of polycondensation or to an intrinsic viscosity degree, sufficiently high to obtain the desired mechanical properties, and especially the acetaldehyde formed during the course of the polycondensation in molten medium, as also to eliminate the degradation products for the most part. By this method it is possible to obtain polyester resins containing less than 2 ppm, and even less than 1 ppm, of acetaldehyde. However, this stage of solid phase polycondensation requires special equipment and consumes a lot of energy. In addition, the use of polyester resins of high intrinsic viscosity, and therefore of reduced fluidity in a molten medium, penalizes the injection cycles- (duration between the start of the injection and the ejection of the piece from the mold) . However, there was apparently some difficulty in proposing a polyester resin that met the requirements related to acetaldehyde, without the need to resort to a solid phase polycondensation stage, and therefore to use a polyester of low degree of polymerization and with a weak tenor of acetaldehyde as well as a formation of acetaldehyde with a speed as small as possible during the injection stage in order to allow obtaining preforms and bottles that meet the specifications regarding its content of acetaldehyde. One of the main objectives of the present invention, is to correct these drawbacks for which proposes a polyester resin of low degree of polycondensation and low tenor of acetaldehyde, which allows to obtain bottles suitable especially for the packaging of liquid food goods such as soft drinks, natural waters or gaseous mineral waters or not . For this purpose, the invention proposes a polyester comprising at least 92.5% of the number of recurring units derived from terephthalic acid of aliphatic diols, characterized in that it has an intrinsic viscosity comprised between 0.45 dl / g and 0.70 dl / g, and an acetaldehyde content of less than 3 ppm, preferably less than 1.5 ppm. It is advantageous that this polymer is crystallized in order to prevent the polyester from being sticky or adhesive at a temperature below about 200 ° C. In this way, the polyester can be processed so as to obtain a certain degree of crystallization, being advantageous that it is greater than 15%. It is advantageous if the intrinsic viscosity of the polymer is between 0.45 dl / g and 0.65 dl / g. The term "intrinsic viscosity" (VI) refers to the viscosity of a polymer solution of zero concentration. Said value is calculated according to the following Formula (I) from the determination of a viscosity index (IV) in dl / g, measured in a polymer solution with 0.5 g of polymer / 100 ml of a solvent constituted by orthodichlorobenzene and phenol (50/50) by weight at 25 ° C, in accordance with ISO 1628/5 of June 15, 1986. For the polyesters of the invention, the intrinsic viscosity (V), expressed in dl / g, is calculated according to the following Formula I: VI = -10_1IV2 + 0.94IV + 0.0122 (I) It is advantageous that the polyester is obtained from the terephthalic acid, its asters or a mixture thereof and of aliphatic diols such as ethylene glycol, 1,3-propanediol, or 1,4-butanediol. Preferred monomers are terephthalic acid and ethylene glycol, which make it possible to obtain polyethylene terephthalate, better known from the. PET abbreviation, as previously mentioned. In this document, the term "PET" also covers a homopolymer obtained only from terephthalic acid monomers or their esters such as dimethyl terephthalate and ethylene glycol, and copolymers comprising at least 92.5% by number of units recurrent of ethylene terephthalate. According to a preferred feature of the invention, the polyester comprises at least one crystallization retardant which makes it possible, especially during the course of the cooling of the molded or injected article, such as a preform, to slow or retard the crystallization of the polyester of so as to obtain a crystallization in the form of crystals of very small size, avoiding spherulitic crystallization, and so as to be able to manufacture a transparent article whose walls do not present a haze, halo or haze, and with acceptable mechanical properties. Said crystallization retarding agents are difunctional compounds such as diacids and / or diols added to the monomer mixture before or during the course of polymerization of the polyester. ? The term "crystallization retarding agent" may be mentioned, for example,. diacids, isophthalic acid, naphthalene dicarboxylic acid, cyclohexane dicarboxylic acid, cyclohexane diacetic acid, succinic acid, glutaric acid, adipic acid, azelaic acid, sebasic acid, and, by way of examples of diols, mention may be made of aliphatic diols comprising from 3 to 20 carbon atoms, cycloaliphatic diols of from 6 to 20 carbon atoms, aromatic diols comprising from 6 to 14 carbon atoms and their mixtures such as diethylene glycol, triethylene glycol , the isomers of 1,4-cyclohexane di-methanol, 1,3-propanediol, 1,4-butanediol, 1,5-pentane diol, the (2, 4) -3 methyl pentanediol, the (1,4) -2 methylpentanediol, the. (1, 3) -2,2,4-trimethylpentane diol, the (1,3) -2-ethylhexanediol, the (1,3) -2,2-diethylpropanediol, the 1,3-hexanediol, the 1,4 -di (hydroxy ethoxy) benzene, 2,2-bis (4-hydroxycyclohexyl) propane, 2,4-dihydroxy-1,3,3-tetramethylcyclohexane, 2,2-bis (3-hydroxyethoxyphenyl) ) propane, 2, 2-bis (4-hydroxy-propoxyphenyl) and its mixtures. Diethylene glycol is frequently present inherently in polyesters, since it is formed during the synthesis by condensation of two molecules of ethylene glycol. Depending on the desired concentration - for recurring units that include a diethylene glycol radical (SDR) in the final polyester, either diethylene glycol is added to the monomer mixtures, or the conditions of the polyester synthesis are controlled for to limit the formation of diethylene glycol. It is preferable that the molar concentration of the diethylene glycol in the polyester, based on the amount of moles of diacid monomers, is less than 3.5%, preferably less than 2%, molar. As for the other crystallization retarders, it is preferable that the molar concentration, based on the amount of moles of all the diacids in the mixture of monomers and therefore in the polyester obtained, is less than 7.5%, with the condition that the DEG tenor must be discounted or deducted from said value if it is present. In other words, the total molar concentration of the crystallization retarder should be less than 7.5%, as already mentioned in European Patent No. 41035. Of course, the polyester may comprise a mixture of crystallization retarding agents. , of the acid type and / or diol. Although the procedures for the injection of the preforms and for the blowing of the hollow containers allow to control in particular the speeds of cooling in a manner. to avoid the spherolitic crystallization of the resin, or in the case in which the bottles to be produced would not be translucent, such as, for example, milk bottles, the total concentration of the crystallization retarder can be very small, for example of the order of 1%, and still zero, with the exception of the DEG formed during the course of the polyester synthesis. According to a preferred embodiment of the invention, it is preferable that the polyester of the invention contains less than 4% isophthalic acid and less than 3.5% diethylene glycol, the latter being expressed as a percentage of the molecule's retarder. crystallization with respect to the amount of moles of all the diacid monomers. The subject of the invention is also a process for the manufacture of a polyester according to the invention. Said process comprises a first stage consisting in manufacturing a polyester by means of a polymerization process in a molten medium in order to obtain a resin having an intrinsic viscosity, it being preferable that said intrinsic viscosity be between 0.45 and 0.8 dl / g, more preferable between 0.45 and 0.75 dl / g. In a second stage, the polyester is formed in the form of granules, by any process adapted and known in the technical field of the manufacture of the polymers. Said granules are subjected to a crystallization step that allows to prevent the granules from agglomerating with each other when they are brought to a temperature lower than 200 degrees centigrade, for example between 130 and 200 ° C. In other words, the crystallization stage it must allow at least the formation of a skin or of an external surface sufficiently crystallized to avoid any stickiness when the granules are brought to a temperature of the order of 200 ° C. Said step can be carried out by keeping the granules at a temperature between 120 ° C and 170 ° C or by treating them with hot water, for example with boiling water, at a temperature between 80 degrees centigrade and 100 centigrade, or by any suitable means to achieve the result described. In a third stage, the polyester granules are subjected to a heat treatment for which they are maintained at a temperature comprised between 130 degrees centigrade and 200 degrees centigrade, preferably between 150 degrees centigrade and 180 degrees centigrade, in order to lower the concentration of the acetaldehyde up to a value of less than 3 ppm and of obtaining a polyester having an intrinsic viscosity of less than 0.70 dl / g. According to a feature of the invention, the heat treatment is carried out in the presence of a gas, preferably by sweeping with a gas through the mass of the granules, said gas having a dew point greater than -60. ° C, preferably comprised between -60 ° C and 20 ° C, and, in a preferred embodiment, comprised between -10 ° C and 20 ° C. The gas scavenging is effected either through a bed of the granules , either through a fluidized bed of the granules, it being preferable that the scavenging gas is the gas used for the fluidization of the granules. When the polyester to be treated has an intrinsic viscosity comprised between 0.7 and 0.8 dl / g, it is preferable that the sweeping gas has a dew point between -10 0 C and 20 ° C (limit values included) . In this embodiment, the treatment method according to the invention can comprise two successive stages consisting of a first stage of treatment by means of a gas with a high moisture content, and even of water vapor, in order to cause a decrease of the intrinsic viscosity, followed by a sweep with a dry gas in order to eliminate the acetaldehyde, while limiting the variation of the intrinsic viscosity. It is preferable that the gas suitable for the invention is air, nitrogen, carbon dioxide, or any other gas that does not degrade the polymer. In a preferred embodiment of the invention, the polyester obtained by polycondensation in molten medium has a low intrinsic viscosity, for example comprised between 0.45 and 0.65 dl / g, and the heat treatment can be carried out with a gas having a low dew point, for example between -60 ° C and 0 ° C. During the course of this treatment, the intrinsic viscosity (VI) may increase slightly but it is preferable that it remains below 0, 70 dl / g, preferably less than 0.65 dl / g. It is preferable that this heat treatment be carried out at a temperature between 130 ° C and 200 ° C, for a duration of a few hours to several tens of hours. The duration of the treatment is determined in such a way as to obtain the desired acetaldehyde content and intrinsic viscosity. As indicated above, the treated polyester is obtained by melt phase polycondensation processes, generally in the molten phase under reduced pressure, as described in the technical literature of the case for more than 50 years. this type comprises a first stage of esterification or transesterification in the presence or absence of catalyst. The obtained hydrolyzed or esterified product is then polycondensed under reduced pressure in the presence of catalysts such as the antimony, titanium or germanium compounds, for example. In this step alcohol or water is removed, in order to allow an advance or acceleration of the polycondensation reaction. According to the invention, polycondensation is stopped when the degree of polycondensation or the intrinsic viscosity have reached the desired value, lower than 0.8 dl / g, preferably lower than 0.75 dl / g. The obtained polyester is cast in rows that allow obtaining rushes or filaments that are then transformed into granules by cutting them. Nevertheless, to allow the formation of these reeds, it is preferable that the polyester have a sufficient intrinsic viscosity, for example greater than 0.45 dl / g. According to another embodiment of the invention, in order to limit the evolution of the degree of polycondensation during the course of the thermal treatment in order to reduce the content of acetaldehyde, the polyester can comprise a monofunctional monomer, preferably a monoacid. The molar content of the monofunctional monomer is between 0.5% mol and 3% with respect to the total of the diacid monomers. Thus, the monoacids suitable for the invention are for example benzoic acid, naphthalene acid, aliphatic acids having a boiling point compatible with the process for the synthesis of the polyester, ie, preferably at least higher than that of ethylene glycol or that of their esters or that of alcohols such as cyclohexanol or aliphatic alcohols, it being preferable that they also have a boiling point higher than that of ethylene glycol. Low-granules of acetaldehyde, obtained by the process of the invention and having an acetaldehyde content and an intrinsic viscosity according to the invention, are preferably used as raw materials for the manufacture of hollow containers, such as bottles. It is possible to add to the polyesters of the invention, either in the polymerization stage or in the molten polyester before injection, different additives such as bluing agents, colorants or other additives stabilizing against light, heat, or antioxidants, for example . These granules are preferably dried, in order to obtain a lower moisture content. at 50 ppm, more preferably still, below 20 ppm. This drying step is not mandatory if the moisture content in the polyester is sufficiently reduced. The granules are then introduced into the injection-blow molding processes for the manufacture of hollow containers such as bottles. These procedures, which have been described in numerous publications and are used industrially on a large scale, cover a first stage of injection for the manufacture of the preforms. In a second stage, the preforms, cooled or not, are reheated to be subjected to a blowing process with which they are transformed into the desired bottles, with possibly a bi-stretch (bi étirage). The preforms are obtained for example by melting the resin in a single or double screw injection press, which also makes it possible to obtain a plasticization of the polyester and to introduce it under pressure in a distributor provided with heated nozzles, for example at a temperature comprised between 260 degrees Celsius and 285 degrees Celsius. The resin is injected into at least one mold of the preform provided with cooling means suitable for controlling the cooling rate of said preform and thus avoiding a spherulitic crystallization, which allows to obtain, preferably if this result is desired. , a preform that does not present a veil or haze on its walls, or opaque walls. After cooling, the preform is ejected and cooled to room temperature, or directly introduced, without cooling, into a blow-off facility such as that described above. In this process of manufacturing the preforms, the polyester is melted at a temperature of the order of 280 degrees centigrade, for example between 270 and 285 degrees centigrade, after which it is injected into the molds. The lowest possible injection temperature shall be used in order to limit the formation of acetaldehyde, in particular to reduce the rate of acetaldehyde formation. In the manufacturing process of the preform, preference is given to an injection pressure for the molten resin in the mold or in the molds., between 2.5 x 10E7 Pa (250 bar) and 5 x 10E7 Pa (500 bar) for a temperature between 250 degrees centigrade and 270 degrees centigrade. This pressure range is smaller than that used for the polyester resin having a higher intrinsic viscosity, namely, greater than 0.70 di / g.
In addition, it is preferable that the molds be cooled to a temperature between 0 degrees centigrade and 10 degrees centigrade. This cooling is obtained by using any suitable cooling fluid such as, for example, water mixed with glycol. It is preferable that the injection and cooling cycle be of the order of 10 seconds to 20 seconds. The polyester forming the wall of the preform obtained by this process has an intrinsic viscosity comprised between 0.45 dl / g and 0.70 dl / g, preferably between 0.45 dl / g and 0.65 dl / g. The content of acetaldehyde in the preform is less than 10 ppm, preferably less than 6 ppm. The preforms thus obtained are generally used in the blowing processes for the manufacture of bottles. These blowing methods are well known and have been described in numerous publications. They generally consist in introducing the preform into a blow-off facility, with or without an over-stretching comprising heating means. The preform is heated to a temperature at least higher than Tg (vitreous transition temperature) of the polymer, after which it is pre-blown by injecting a gas under pressure at a first pressure during a first period. A second injection of a gas under a second pressure allows to obtain the final shape of the bottle before its ejection after cooling. It is preferable that the heating temperature of the preform be between 80 ° C and 100 ° C. This heating is carried out by any suitable means, for example by infrared radiation directed towards the outer surface of the preform. It is preferable that the pre-blowing of the preform takes place at a first pressure comprised between 4 x 10E5 Pa and 10 x 10E5 Pa (between 4 bar and 10 bar) for a period between 0.15 and 0.6 seconds. The second blowing is carried out under a second pressure comprised between 3 x 10E6 Pa and 4 x 10E6 Pa (between 30 and 40 bar) during a second period comprised between 0.3 and 2 seconds. As is known, it is also possible to introduce a stretching rod into the preform during the pre-blowing and / or blowing operations, in order to partially stretch the preform. The use of a polyester according to the invention makes it possible to obtain hollow bottles or containers which have a reduced content of acetaldehyde. Therefore, one of the objects of the invention relates to bottles whose polyester forming the walls has an intrinsic viscosity comprised between 0.45 and 0.70 dl / g and an acetaldehyde content of less than 6 ppm. In addition, this material allows the manufacture of bottles of equal content, with mechanical properties at least equivalent to those obtained with a polyester having a higher intrinsic viscosity. The invention also relates to hollow containers such as bottles obtained with a polyester according to the invention and following the injection-blow molding methods described in the present application. However, hollow containers obtained by shaping a polyester according to the invention, said shaping being carried out by methods other than those described in the present application, also form part of the present invention. The bottles of the invention are used for the packaging of any liquid product, more particularly for the packaging of liquid food products such as sugary drinks, soft drinks or not, known under the general designation "soda"., The different natural, thermal waters or minerals, soda or not.
Other detailed advantages of the invention may be appreciated by the examples given below only by way of indication. In the following examples, the determination of the concentration of acetaldehyde is carried out by means of the following analysis protocol: ".- a sample of granules (approximately 4 g) is crushed in a cryogenic crusher under liquid nitrogen, obtaining a powder that passes through a sieve of 800 pm mesh (example: crushing in a Spex 6700 brand shredder that works for two minutes with the frequency inverter placed in minimum position and for five minutes with the frequency inverter placed in maximum position) and preferably having a granulometric curve comprised between 200 and 400 pm (the grinding must be carried out under conditions that do not degrade the polymer). .- an amount of approximately 500 mg, accurately weighed, of this powder, is arranged in a tube with a top dead space ("head space"), closed. . - this "head space" tube is placed in a vapor phase chromatography analyzer (Nhead space): Perkin Elmer HS 40 and chromatograph: VARIAN 3500 with a PORAPLOTQ column and a helium vector gas with a flow rate of 5 ml / min) and a temperature that varies between 60 and 230 ° C (with a gradient of ascent of + 10 ° C / min) .- The "head-space" tube is maintained for 90 minutes at a temperature of 145 ° C. The calibration of the chromatograph is carried out by means of acetaldehyde solutions in dimethylacetamide Example 1 In a polycondensation facility comprising a 7.5 1 reactor, a copolymer containing 2.3 mol% of isophthalic acid referred to the all of the diacid monomers, as a crystallization retarding agent, there being other monomers present such as terephthalic acid and ethylene glycol, and using 250 ppm, expressed as antimony, of antimony oxide as a catalyst. The condensation is stopped when the intrinsic viscosity of the polyester is 0.48 dl / g and contains 3.4 mol% of DEG with respect to the number of moles of diacid monomers. The polymer is formed into granules by extrusion in the form of reeds that are cut out, whereby a granulate having an average mass of 13 milligrams per granule is obtained. The residual acetaldehyde content in the amorphous granules, determined by the method just described, is 140 ppm. The granules are subjected to an oven crystallization for 30 minutes at 160 degrees centigrade, then they are arranged in a column. A flow of dry air (dew point less than -60 ° C) is passed through the column with a flow rate of 0.39 Nm3 / h for 8 hours, maintaining the temperature of the granules at 180 degrees Celsius. After a treatment with a duration of eight hours, the polyester has an intrinsic viscosity of 0.56 dl / g and an acetaldehyde content of 0.75 ppm. This polyester resides the designation of "Polyester A". Eloos 2 to 5 A polyester containing 2.3 mol% of isophthalic acid is prepared, following an identical operating mode to that of Example 1. However, the polycondensation was interrupted when the intrinsic viscosity was equal to 0.64 dl / g . The amorphous granules have an acetaldehyde content equal to 45 ppm and 3.4 mol% of DEG. The granules have been subjected to crystallization, for which they were kept for 30 min in an oven at 160 ° C.
The granules have been subjected to treatments for the purpose of removing the acetaldehyde, with various gases and at different temperatures, according to the operating mode described in Example 1. The treatment conditions, the acetaldehyde contents, and the viscosity values intrinsic, they have been consigned in the following Table 1: Table 1: Example Gas Tempera Duració Fear VI Polymer-nt of AA (dl / g) o. (hours) (ppm) 2 (1) Air 183 ° C 10, 30 0.8 0, 60 B dry 3 Air 185 ° C 8 1.3 0, 63 C (dew point: -10 C) 44 Air 185 ° C 8 1.5 0, 60 D (spray temperature: +18 C) 5 Air 200 ° C 8 1.6 0.56 E (dew point: +18 C) (1) .- a pre - treatment with water vapor instead of air, at the beginning of the procedure, for half an hour Polymers B to E obtained in these examples have a better coloration. Said improvement is translated into a decrease in the yellowness index. The presence of moisture makes it possible to limit the degradation of the yellowness index of the polyester. Example 6: A homopolyester was prepared by adding 2.6 mol% of benzoic acid referred to terephthalic acid, for which the operative modality described in Example 1 was followed.
The polycondensation was interrupted when the intrinsic viscosity reached a value of 0.47 dl / g. After the granulation, the obtained polymer contains 70 ppm of acetaldehyde and 3.5 mol% in DEG. After crystallization and treatment by dry air (dew temperature: -60 ° C) at 180 ° C for 8 hours, following the operative mode of Example 1, the granules have an intrinsic viscosity of 0.53 dl / g and a content of acetaldehyde of 1.6 ppm. Said polymer receives the denomination of polymer F. Examples 7 to 9 For the manufacture of bottles according to the following procedure certain polyesters obtained based on the examples just described were used: The polyester granules are melted in an endless mono-screw whose casing temperature is 285 ° C. The molten polyester is fed into a pre-form injection apparatus marketed under the designation "Presse HÜS and 48 imprints XL 300" with an injection temperature of between 262 ° C and one pressure of 450 bar. The cooling of the pre-forms is carried out by circulating water at a temperature of 8.5 ° C. The overall cycle time for the injection is 15, 7 ... seconds.
After the pre-forms are cooled, they are introduced in a blowing installation for the manufacture of bottles with a capacity of half a liter having a neck or neck whose form resides the standardized denomination of 28 PCO; the bottom of these bottles has a petaloid shape with 5 petals. Said installation is marketed under the designation "SIDEL SBO 1 F2 Lab". The pre-forms are heated to a temperature indicated in the following table. The pre-blowing is carried out for 0.19 second under a blow pressure of 8.5 bars. The blowing is carried out for 1.78 seconds under a blowing pressure of 38 bar. The speed of the stretching rod is 1.2 m / sec. The characteristics of the bottles obtained and of the blowing process have been reported in the following Table for each of the polyesters used. - Table II Example 7 8 9 · Polyester ABF Pre-temperature 87 ° C 96 ° C 89 ° C shape Burst pressure 13,84 bar 15,68 bar 13 84 bar drawing index 11, 9 11,9 11 , 9 the plane Highlight Density 1, 363 1.3625 1, 364 (g / cm3) Medium 1, 3615 1, 3632 1, 363 Background 1, 3612 1, 3643 1, 3622
Acetaldehyde content 2.85 ppm 3.05 ppm 3.00 ppm The viscosity of the polyester forming the walls of the bottle is of the same order as that of the polyesters used.